Sheikh Muhammad Al-Yaquobi, former preacher of the Grand Umayyad Mosque of Damascus, leads funeral prayers for Peter Kassig in the mosque at Al-Huda Foundation in Fishers, Ind., Friday, Nov. 21, 2014. (Michael Conroy/AP)

Muhammad al-Yaqoubi was the preacher of the Grand Umayyad Mosque in Damascus until his exile in 2011.

I recently traveled from Morocco to Fishers, Ind., to lead funeral prayers for Peter Kassig, an American aid worker killed by terrorists from the so-called “Islamic State.” On behalf of the Syrian people, I offered my condolences to Kassig’s parents and extended my thanks for his heroic efforts. It was the least I could do for a courageous American who gave up his comfortable life in the United States to serve desperate civilians in war-torn Syria.

It is unfortunately not surprising that the Islamic State brutally beheaded Peter — who was also known as Abdul-Rahman Kassig, the name he took after his conversion to Islam following his capture — even as he was fulfilling the prophet Muhammad’s directive to “show mercy toward those on earth, so that you may receive mercy from heaven.” His killing goes against every Islamic principle that the Islamic State claims to embrace.

The Islamic State is likely the most extreme group in Islam’s history. It can be compared to the 7th-century Kharijite sect, which zealously executed even fellow Muslims for apostasy to the extent that Muslim travelers through Kharijite lands would pretend to be pagans seeking conversion. The Islamic State likewise brutally slaughters devout Muslims for their advocacy of democracy, although the majority of Sunni scholars have said that democracy is compatible with Islam. The group also engages in sectarian cleansing of Yazidis, Christians and those of other faiths, despite well-established Sunni jurisprudence protecting the rights of non-Muslims.

Sunni Muslim scholars across the world have condemned the Islamic State’s crimes and sought to rebut its flimsy arguments. I joined more than 100 other prominent Islamic scholars to issue a fatwa, or Islamic edict, that refutes the deviant Islamic State ideology from the perspective of Islamic jurisprudence, showing how the group has twisted the words of the Holy Koran and the prophetic tradition. In a statement last month, I further noted that Muslims have a moral duty to warn law enforcement agencies if they know of anyone traveling to join the Islamic State.

However, more needs to be done. To defeat the Islamic State, we need to take serious steps to deprive it of the political justification for its existence on Syrian soil: the regime of dictator Bashar al-Assad.

For centuries, my home town of Damascus was renowned as a beacon of religious coexistence among Sunnis, Shiites, Jews and Christians. In 1944, almost two decades before the United States passed the Voting Rights Act and more than a half-century before Americans elected Barack Obama as president, Syrians chose Fares al-Khoury, a Protestant Christian, as their prime minister.

The Syrian uprising of 2011 was based on a desire to return to our grand past. It was a protest movement of all faiths, as Syrians of all stripes took to the streets chanting for freedom and democracy. But the Assad regime cracked down with unspeakable horrors. More than 200,000 people have been killed as the regime deployed its full arsenal, including barrel bombs and sarin gas, against civilians. More than 9 million Syrians have been displaced, including more than 3 million refugees, and thousands have been tortured to death in Assad’s dungeons. All this occurred while the world looked on.

If the West truly wants to destroy the Islamic State, it should force the Assad regime, through diplomatic and military pressure, including airstrikes against the regime’s power centers, to negotiate a political transition by which it hands over power so that the Syrian people can unite to rebuild the country and fight terrorism.

The Islamic State has benefited from the world’s inaction; had it not been for the tyrannical Assad regime and its genocidal campaign against its people, we would not have seen foreign fighters pouring into Syria or witnessed the group exploit the chaos and suffering to develop its own state apparatus. Further inaction in toppling the regime will threaten the entire region and the United States.

Peter Kassig, a hero to the Syrian people, risked his life to provide healing and hope to Syrians amid the carnage and despair. For the sake of his noble sacrifice, we are duty-bound to defeat the Islamic State, end Assad’s reign of terror and continue the quest for healing.